Plastics foams are synthetically produced materials with cell structure throughout their entire volume, with a density lower than that of the compact material from which they were produced. Their importance as lightweight construction material is constantly increasing, because they often have an unusual combination of mechanical properties with low densities. Thermal conductivity values are often very low, and therefore some foams have considerable importance as insulation material.
The disposal of plastics foams after use thereof is sometimes problematic. Another possibility often also available alongside energy recycling is materials recycling. However, biodegradability by way of example in a composting system would be desirable.
Polyester-based foams are known from various publications. These foams are usually produced by an extrusion process.
Said process has the disadvantage that it can generally produce only simple shapes such as blocks, sheets, and thin layers (foam foils). There is also often a restriction on the maximum thickness of blocks that can be produced by this process.
In contrast, moldings of almost any desired geometry can be produced by using foam beads. WO 2012/020112 describes by way of example the production of expandable polyester-based beads and foam beads obtainable therefrom by use of an extrusion process. However, the mechanical properties of moldings produced therefrom are not always suitable for applications which require high tensile strengths and high tensile strain at break.
These problems can be avoided by using what is known as the autoclave process as described by way of example in Ullmann's Encyclopedia of Industrial Chemistry, vol. A11 1988. The range of requirements placed upon the foam beads thus produced and the moldings resulting therefrom can differ greatly, depending on application sector. However, it is possible to define common minimum requirements for the suitability of the process for various materials.
These are:
                The capability to produce foam beads over a wide range of bulk density, and in particular here it should be possible to achieve a very low bulk density irrespective of bead geometry and size. A low weight of the resultant moldings leads to cost savings.        Foam beads with predominantly closed-cell foam structure. This leads to good processibility in commercially available molding machines using the pressure-fill method, and to low water absorption.        Complete impregnation of the polymer beads to give foam beads without compact core. (No impairment of mechanical properties or of tactile properties of the resultant moldings.)        
Numerous parameters can be varied in the autoclave process in order to comply with these requirements. These are inter alia the suspension medium, the type of blowing agent and concentration thereof, the heating curve, and the impregnation temperature (IMT). It is not generally possible here to take a parameter, or parameter combination, that is particularly suitable for one material and use it for another material.
WO 00/68303 describes in general terms the production of expanded polymer beads based on biodegradable saturated polyesters in an autoclave process. Suspension medium preferably used here is ethylene glycol and glycerol with a density of from 1.1 to 1.25 g/cm3. These processes are not always entirely satisfactory in respect of the procedure, the capability of the process, and the property profile of the expanded foam beads thus obtainable.
JP 2004 143269 discloses an autoclave process which foams micropellets made from the biodegradable polyesters defined at the outset; the advantageous cooling of suspension medium by adding a coolant is not described in JP 2004 143269. With regard to the batch homogeneity, the surface properties of the foam beads and the associated bead gloss, the process known from JP 2004 143269 is unsatisfactory. Furthermore, the foam beads obtainable from micropellets and the moldings obtainable therefrom have unsatisfactory rebound.